The present invention relates generally to plastic bags. More particularly, the present invention relates to the use of particular polymers in food bags to strengthen the fins and fin seals that attach the zipper to the bag.
The use of plastic bags is increasingly more common in the marketplace. A factor affecting their acceptance and range of application, however, is the strength and reliability of the bags and the seals.
Plastic bags are in widespread use in a varied and diverse number of household and commercial applications. The primary advantage of plastic bags is their ease of opening and resealing. The plastic bag includes first and second opposing body panels fixedly connected to each other along a pair of sides and a bottom, in which the bottom extends between the pair of sides. The plastic bag includes a fastener, such as a zipper, extending along a mouth formed opposite the bottom of the plastic bag. The zipper includes a male track and a female track. The male track includes a male profile and a first depending fin or flange extending downward from the male profile. Likewise, the female track includes a female profile and a second depending fin or flange extending downward from the female profile. The first and second fins may be extruded separately from the body panels and then thermally fused to inner surfaces of the respective first and second body panels.
The male and female tracks may be rolled or pressed into their interlocking arrangement so as to securely close the bag by one of two means. First, the tracks are rolled or pressed together at one end by a user and then sequentially fitted together along the length of the zipper by the user running a finger along the length of the zipper on each side of the tracks. Alternatively, some bags employ a plastic slider that rides along the tracks of the zipper. If the slider is pulled in one direction, the bag is sealed shut; if the slider is pulled in the opposite direction, the bag is reopened.
Plastic bags with a zipper-type closure mechanism typically have at least two types of seals. One type of seal (xe2x80x9cside sealxe2x80x9d) seals a first body panel to a second opposing body panel along the sides of the plastic bag. A second type of seal (xe2x80x9cfin sealxe2x80x9d) is created when a plastic fin is used to attach the zipper-type closure mechanism to the body panels. Since the popularity of these bags has placed increased demands on the tasks they are asked to perform, a need exists for bags having fin seals that are able to withstand increased forces and more adverse conditions.
It is an object of the present invention to create plastic bags that have fins and fin seals that are able to withstand greater forces and more adverse conditions than previous bags.
The present invention improves performance of the fin seal by making the fin out of resins that have a narrow molecular weight distribution or polydispersity (Mw/Mn or MWD). The inventive fin comprises a first resin and, optionally, a second resin.
According to one embodiment of the present invention, a zippered plastic bag has first and second panels, a zipper and a slider. The zipper includes a male and female track. The male track includes a male profile and a first fin portion. The first fin portion is affixed to the first panel in proximity to the top of the first panel. The female track includes a female profile and a second fin portion. The second fin portion is affixed to the second panel in proximity to the top of the second panel. The male and female profiles have complementary cross-sections. The first and second fin portions are attached. At least one of the fin portions is made from a first resin which is prepared in the presence of a single site catalyst and a second resin. The first resin has a polydispersity of from about 2 to about 3, a melt index of from about 0.2 to about 20 g/10 min., and a melt flow ratio of from about 12 to about 35. The second resin is a low density polyethylene. At least one of the fin portions comprises from about 5 to about 50 wt. % of the first resin, and from about 50 to about 95 wt. % of the second resin.
According to another embodiment of the present invention, a zippered plastic bag has first and second panels, a zipper and a slider. The zipper includes a male and female track. The male track includes a male profile and a first fin portion. The first fin portion is affixed to the first panel in proximity to the top of the first panel. The female track includes a female profile and a second fin portion. The second fin portion is affixed to the second panel in proximity to the top of the second panel. The male and female profiles have complementary cross-sections. The first and second fin portions are attached. At least one of the fin portions is made from a first resin selected from the group consisting of an ultra low density polyethylene, a very low density polyethylene, a metallocene-catalyzed linear low density polyethylene, and an elastomer. The first resin has a polydispersity of from about 1 to about 4, a melt index of from about 0.2 to about 20 g/10 min., and a melt flow ratio of from about 12 to about 35, and a second resin is a low density polyethylene. At least one of the fin portions comprises from about 5 to about 50 wt. % of the first resin, and from about 50 to about 95 wt. % of the second resin.
According to a further embodiment of the present invention, a fastener comprises a male track and the female track. At least one of the fin portions is made from a first resin which is prepared in the presence of a single site catalyst. The first resin has a polydispersity of from about 2 to about 3, a melt index of from about 0.2 to about 20 g/10 min., and a melt flow ratio of from about 12 to about 35. A second resin is a low density polyethylene. At least one of the fin portions comprises from about 5 to about 50 wt. % of the first resin, and from about 50 to about 95 wt. % of the second resin. Alternatively, at least one of the fin portions comprises from about 50 to 100 wt. % of the first resin, and from 0 to about 50 wt. % of the second resin.
Alternatively, at least one of the fin portions of the fastener may be made from a first resin selected from the group consisting of an ultra low density polyethylene, a very low density polyethylene, a metallocene-catalyzed linear low density polyethylene, and an elastomer. The first resin has a polydispersity of from about 1 to about 4, a melt index of from about 0.2 to about 20 g/10 min., and a melt flow ratio of from about 12 to about 35, and a second resin is a low density polyethylene. Alternatively, at least one of the fin portions is made from an ultra low density polyethylene, a very low density polyethylene and a metallocene-catalyzed linear low density polyethylene comprising from about 50 to about 90 wt. % of the first resin, and from about 10 to about 50 wt. % of the second resin. The fastener may also have at least one of the fin portions made from an elastomer comprising from about 50 to 100 wt. % of the first resin, and from 0 to about 50 wt. % of the second resin.
According to yet another embodiment of the present invention, a zippered plastic bag has first and second panels, a zipper and a slider. At least one of the fin portions is made from a first resin which is prepared in the presence of a single site catalyst. The first resin has a polydispersity of from about 2 to about 3, a melt index of from about 0.2 to about 20 g/10 min., and a melt flow ratio of from about 12 to about 35. A second resin is a low density polyethylene. At least one of the fin portions comprises from about 50 to 100 wt. % of the first resin, and from 0 to about 50 wt. % of the second resin.
According to another embodiment of the present invention, a zippered plastic bag has first and second panels, a zipper and a slider. At least one of the fin portions is made from a first resin that is an ultra low density polyethylene, elastomer, metallocene-catalyzed linear low density polyethylene, or a very low density polyethylene. The first resin has a polydispersity of from about 1 to about 4, a melt index of from about 0.2 to about 20 g/10 min., and a melt flow ratio of from about 12 to about 35. A second resin is a low density polyethylene. At least one of the fin portions comprises from about 50 to about 90 wt. % of the first resin, and from about 10 to about 50 wt. % of the second resin.
According to one aspect of the invention, the first resin is a polymer having a narrow molecular weight distribution or polydispersity of from about 1 to about 4, a melt index of from about 0.2 to about 20, and a melt flow ratio (I20/I2) of from about 12 to about 35. Examples of the first resins are linear low density polyethylenes (LLDPE) and metallocene-catalyzed LLDPE. Use of narrow molecular weight distribution resins, like the first resin, results in fin seals which are stronger, tougher, and less likely to leak. The second resin is a polymer, such as low density polyethylene (LDPE), LLDPE, or a blend thereof.
In fins comprising both a first and a second resin, the first resin may be coextruded with a second resin as a thin layer of the fin or may be blended with the second resin. Preferably, the fins are formed entirely of the first resin. In one embodiment, a thin layer of the first resin having a narrow molecular weight distribution is coextruded with at least one of the following of the second resins: LDPE resin, LLDPE resin, or a LDPE/LLDPE blend. The term xe2x80x9clayer,xe2x80x9d as used herein, shall include any coating, film, lamination coextrusion, or the like.